FISHES OF DOUGLAS LAKE, MICHIGAN. 24I  



that may end in leaps into the air. Often the flight is toward shore into water so 

 shallow that the deep-bodied bass are unable to follow. 



Two influences seem to keep the young fish on the sand shoals. The first is 

 abundance of plankton Crustacea, which at this time make up the whole food of the 

 three species. Since these Crustacea no doubt have a uniform horizontal distribution, 

 as in other lakes, their abundance merely permits the shoals to be used; it does not 

 restrict the fish to them. The second influence, that which determines the use of the 

 shoals, is the relative freedom that they afford from attacks of enemies. This freedom 

 aff'orded by the mere presence of the fish on the shoals is increased by their habit of 

 schooling. The coming together of three or four species to fomi a large school lessens 

 the loss to each species and the risk to each individual. This follows in part from 

 the fact that, if the toll taken by enemies remains constant, a large school loses a 

 smaller percentage of its constituents than a small school. It follows also because in 

 a large school there are more eyes on the watch for enemies, and therefore more chance 

 that any individual will be warned by the flight movements of comrades, and thus be 

 enabled to escape. Both the habit of schooling and that of frequenting the shoals are 

 measures of protection, most effective when coincident, for the habit of schooling is 

 most effective in open water, which affords no lurking place for the enemy, and in shal- 

 low water in which an enemy's approach is limited to the lakeward side. It would 

 probably be ineffective or harmful in vegetation. That the occurrence of young fish 

 on the shoals may be further influenced by temperature is indicated by the observa- 

 tions of Michael (Hankinson, 1908, p. 202-204). 



By early September the young perch have reached a length of somewfiat more than 

 2 inches. The suckers and spot-tailed minnows are of the same size. A year later the 

 young perch have reached a length of about 4 inches, have changed their food habits, 

 and are found in the vegetation. They have left the shoals and the community of young 

 fishes and have now become for a time a part of the vegetation community. They 

 do not return to the shoals, but their place there is taken by young fish of the year. 

 The migration from the shoals probably takes place in late fall or early winter, so soon 

 as low temperature or ice makes them uninhabitable, and when the perch are prob- 

 ably less than 3 inches long. The spot-tailed minnows also pass from the shoals into 

 the vegetation and are thenceforth a part of its community. The young suckers, too, 

 leave the shoals, and our next knowledge of them is when they are about 7 inches long. 

 They are then ranging far beyond the vegetation into deeper water. Whether they are 

 confined to the vegetation when between 2 or 3 and 7 inches in length we do not 

 know. It is clear that the communities of young fishes are temporary. 



The community of young fishes is found on the stony shoals as well as on the 

 sand shoals, but on the stony shoals there is added to it the blunt-nosed minnow. The 

 adult males of this species burrow beneath stones or other objects on the bottom, and 

 thus form nests (Hankinson, 1908). The eggs are attached to the lower sides of the 

 stones. The young fish are added to the commimity last described. Nothing is known 

 of their food in Douglas Lake. It is presumably plankton Crustacea, the same as 

 that of the other young fishes of the community, and they are doubtless held to the 

 community by the same factors. The adults have been found only on or near stony 

 shoals. 



